The overall objectives of this proposal are to determine the prevalence rate of true cryptorchidism during early infancy and to assess the role of maternal and gestational factors in the epidemiology of cryptorchidism. Specifically, this study will determine the frequency of crytorchidism at birth, at ages 3 months and one year of age. To accomplish this, approximately 5,800 consecutive singleton liveborn male infants delivered at Mount Sinai Hospital will be examined for cryptorchidism at birth. Those who are found to be cryptorchid at birth will be followed up at 3 months and one year of age to assess the frequency of spontaneous testicular descent and provide an estimate of the proportion of infants with true cryptorchidism. A case-control study will be undertaken to investigate epidemiologic risk factors of cryptorchidism. More specifically, this study will assess whether: a) the apparent excess risk of testicular cancer among whites as compared to blacks is reflected in a racial/ethnic differential in the occurrence of cryptorchidism for white, black and Hispanic infants, b) cryptorchidism is associated with intrauterine growth retardation and factors implicated in intrauterine growth retardation, such as cigarette smoking and other substance abuse, and c) cryptorchidism is related to previously suggested risk factors, such as maternal obesity, prenatal exogenous hormone administration, a history of habitual abortion, and breech delivery. The final study groups will comprise an estimated 150 infants whose testes are undescended at age one (the cases) and 370 infants whose testes are descended at birth (the controls). A third group, which will also be compared to the controls, will consist of approximately 180 infants whose testes descend spontaneously between birth and one year of age. Information on the risk factors will be obtained by a questionnaire administered to the mothers during the postpartum hospital period. This study should provide important information on the epidemiology and pathogenesis of cryptorchidism and potentially on the causes of testicular cancer since cryptorchidism is the major known risk factor for testicular cancer in young men.